The present invention relates generally to liquid crystal cells and more particularly to specifically configured liquid crystal cells especially suitable for use in light responsive, transmissivity variable eyewear, to the eyewear itself and to different methods of making the cells.
Light responsive, transmissivity variable eyewear is known in the prior art. One example is diagrammatically illustrated in FIG. 1 and generally indicated by the reference numeral 10. Eyewear 10 is shown including a frame 12 which contains spaced apart liquid crystal cells 14, one of which will be described in more detail immediately below in conjunction with FIG. 2. These cells are driven by circuitry including a solar cell 16 supported by frame 12 such that the transmissivity of each cell 14 varies inversely to at least a limited extent with the amount of light reaching the solar cell. Thus, under low light ambient conditions, for example at night or in a dark restaurant, cells 14 will be highly transmissive and outdoors on a sunny day the cells will darken and function as sunglasses.
FIG. 2 diagrammatically illustrates one of the cells 14 in detail and the circuitry necessary to drive it in the manner described above. Specifically, as seen in FIG. 2, each cell 14 includes a pair of spaced apart transparent substrates 18 in confronting parallel relationship to one another so as to include outer surfaces 20 and inner confronting surfaces 22 which are uniformly spaced apart from one another along the entire extent of the substrates. For purposes of clarity, the two substrates 18 have been shown much thicker than they actually are and the spacing has been exaggerated. In addition, while they are in confronting parallel relationship to one another, this does not preclude the substrates from defining gentle or even somewhat abrupt curves, depending upon the application of the eyewear. In any case, transparent electrodes 24, again exaggerated in thickness for purposes of clarity, are disposed over the inner surfaces of substrates 18 and, at the same time, they are connected to a voltage control arrangement 26 which includes previously recited solar cell 16. Voltage control arrangement 26 is designed to apply a voltage across the substrates in a direction normal to their inner and outer surfaces which is a function of the amount of light that reaches solar cell 16 so that, in the absence of light, no electric field is present.
Still referring to FIG. 2, a guest-host liquid crystal/dye mixture 28 is contained between the inner confronting surfaces of the substrate, which surfaces define a spacing therebetween for receiving the liquid crystal/dye mixture. The spacing is sealed by suitable means not shown and is maintained uniform in thickness along the entire extent of the substrates by suitable spacing members, two of which are shown at 30 in the form of transparent spacer balls. The means of spacing is not limited to the use of spacer balls. The guest-host liquid crystal/dye mixture includes liquid crystal molecules having negative dielectric anisotropy and defining long axes and dichroic dye molecules which also define long axes and which preferentially absorb light that is polarized along their long axes. The liquid crystal making up this mixture may be, for example, cholesteric (TNLC) liquid crystal as described in Senatore U. S. Pat. No. 5,067,795.
In addition to the components described thus far, each liquid crystal cell 14 includes homeotropic alignment means 32 disposed on the inner confronting surface of each of the substrates 18 for acting on the guest-host liquid crystal/dye mixture (i) such that, in the absence of an electrical field across the substrates, the long axes of the liquid crystal and dye molecules line up relative to the substrates in a way which causes the dye molecules not to absorb most of the light that is directed through the guest-host liquid crystal/dye mixture normal to the substrates, whether the light passing therethrough is polarized or not whereby the liquid crystal cell remains in a relatively clear state in the absence of the electric field, and (ii) such that in the presence of the electric field across the substrates, the long axes of the liquid crystal and dye molecules line up relative to the substrates in a way which causes the dye molecules to absorb at least some light that is directed through the guest-host liquid crystal/dye mixture normal to the substrates, so long as the last mentioned light is polarized along the long axes of the dye molecules, whereby the liquid crystal cell darkens in the presence of the electric field. In the case of voltage control arrangement 26 including solar cell 16, outside at night or in a dark room, the solar cell will not be exposed to light and therefore no electric field will be present and, hence, eyewear 10 will function in its clear state. On the other hand, outside during the day in the sunlight, the eyewear will function as sunglasses.
One problem with the liquid crystal eyewear of the prior art, specifically sunglasses, relates to their achievable contrast ratio as they switch between clear and dark states. Specifically, heretofore, the liquid crystal sunglasses Applicants are aware of display a relatively low contrast ratio, which means that the difference between their clear and dark states is relatively small. Moreover, regarding these sunglasses Applicants are aware of their clear state is not particularly clear so that the end product is relatively dark in the clear state and relative clear in the dark state, which of course is the worst of all worlds, at least as for as Applicants are concerned.
Having described prior art eyewear 10 generally, attention is now directed to a specific example of prior art eyewear.
A specific example of eyewear in the prior art is described in the previously mentioned Senatore U. S. Pat. No. 5,067,795. There, the nematic liquid crystal having a negative dielectric anisotropy serves as a host to a guest dye; homeotropic alignment is provided to align the cell in the unpowered state; and the liquid crystal becomes cholesteric (TNLC) in the powered state. In this design, the product of the pitch in the cholesteric and the birefringence (.DELTA.n, called the double refraction in the patent specification) must be much less than a wavelength. This is necessary to avoid adiabatic following of the helix, and hence poor optical absorption. Another constraint is that the pitch must not be too short or the homeotropic (unpowered) state becomes unstable and consequent optical scattering will result. The result of these tradeoffs is that in the Senatore patent, it is stated that the pitch must be 0.9-4 times the cell thickness and the birefringence must be less than 0.12 in the cholesteric phase. Moreover, as is evidenced in Senatore, its reported contrast ratio is not particularly high.
As will be seen hereinafter, Applicants have designed eyewear having a number of advantages including a relatively high contrast ratio in which the clear or light state is substantially clear and the dark state is relatively dark, that is the best of all worlds as for as Applicants are concerned.